Hydraulic drive, in particular for two-cylinder thick matter pumps

ABSTRACT

The invention relates to a hydraulic drive, especially for two-cylinder thick matter pumps. Said hydraulic drive comprises at least one main pump ( 10 ) embodied as a hydraulic variable displacement pump, and two hydraulically actuatable drive cylinders ( 20, 22 ) that are each connected to a connection line ( 16, 18 ) of the main pump ( 10 ) by means of pump connections mounted on one end thereof, forming a closed hydraulic circuit, and communicate with each other at the ends thereof opposing the pump connections ( 24, 26 ) by means of a swinging oil line ( 28 ). The hydraulic drive also comprises a feed pump ( 42 ), by which means pressurized oil is fed from an oil tank to the current low-pressure side into the hydraulic circuit, and a flushing branch ( 52 ), by which means a flushing oil flow is branched off from the current low-pressure side under a limited pressure into the oil tank ( 44 ). In order to avoid low-pressure influxes during the reversal process of the main pump ( 10 ), the flushing oil flow is blocked and then released in a time-delayed manner during each reversal process of the main pump ( 10 ), independently of the differential pressure in the connection lines.

CROSS REFERENCE TO RELATED APPLICATION

This application is a national stage of PCT/EP2006/001432 filed on Feb.17, 2006 and based upon Application No. 10 2005 008 2173.3 filed on Feb.22, 2005 under the International Convention.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention concerns a hydraulic drive, in particular for atwo-cylinder thick matter pump of the general type described in thepre-characterizing portion of claim 1.

2. Description of Related Art

Known hydraulic drives of this type include at least one main pumpembodied as a hydraulic variable displacement pump as well as twohydraulically actuated drive cylinders embodied as piston cylinderunits. The drive cylinders are connected at their one end via pumpconnections with respectively one of two connection lines of the mainpump, forming a closed hydraulic circuit, while on their end opposite tothe pump connections they are in communication with each other via anoscillating hydraulic line. The main pump, which is embodied as areciprocating pump, is further connected with a control mechanism foralternatingly reversing the direction of flow with reciprocal buildingup of high pressure and a pre-tensioned low pressure in the twoconnection lines. Further, a hydraulic feed pump is provided, of whichthe suction inlet is connected to a hydraulic oil tank and the pressureoutlet is set to a predetermined low pressure level and communicateswith the two connection lines of the main pump via respectively oneone-way valve. Since the hydraulic oil heats up during the pumpingprocess, a sump or flushing branch is supplementally provided, which onthe outlet side communicates with the oil tank via a pressure limitingvalve and on the inlet side is respectively connectable with the lowpressure part of the hydraulic circuit. For this purpose there islocated in the flushing branch a reciprocating valve pre-controlled bythe pressure differential existing between the connection lines of themain pump, which in the case that there is a prevailing pressuredifferential, directs flow towards the respective low pressure sideconnection pipe which is accompanied by discharging of a sump streaminto the oil sump tank, and in the case that there is no pressuredifferential is in a blocked intermediate position. The amount of oildischarged during the flushing process corresponds to approximately50-70% of the oil amount continuously re-supplied from the oil tank bythe feed pump. Due to the mass inertia and the compressibility in thesystem, substantial pressure oscillations result within the hydraulicsystem during the reversal process of the reversible pump. During thereversing process the pivot angle of the adjustable pump is retracted.Thereby the volumetric displacement of the reversible pump becomeslower. Since the system is still on line, the high pressure drops whilein equal value the low pressure increases. This means that the lowpressure side experiences a rapid pressure increase so long as thereverse valve in the flushing cycle is not yet redirected. This leads toan extreme discharge of flushing oil from the until now low pressureside of the main circuit so that, in the course of the renewed pressurebuildup on the high pressure side, the pressure on the low pressure sidecan completely collapse within a fraction of a second. The feed pump inthis condition is not able to compensate for and re-supply the flushedamount discharged from the flush circuit. Due to the undersupply on thelow pressure side, pipe clanging and cavitation is produced both in thefeed pump as well as in the main pump, with a danger of an increasedwear and tear. In order to avoid this problem, it has already beenproposed that the deficit in oil is to be compensated by a pressurereservoir or a larger feed pump. Both solutions however require anundesirably high construction investment.

SUMMARY

Beginning therewith it is the task of the invention to make arrangementswhich ensure that the low pressure collapse in the hydraulic drive ofthe above-described type with closed hydraulic circuit is preventedduring the reversal process with simple means.

For solving this task, the combination of characteristics set forth inclaims 1 and 11 are proposed. Advantageous embodiments and furtherdevelopments of the invention can be seen from the dependent claims. Thesolution according to the invention is based primarily upon the idea,that the flushing stream discharged from the hydraulic circuit duringeach of the reversal processes of the reversible pump is blocked for ashort time independently of the pressure differential in the connectionlines of the main pump, and subsequently again are re-opened. In orderto make this possible, there is proposed in accordance with theinvention a supplemental blocking mechanism, which blocks the flushingstream during the reversal process of the main pump independent of thepressure differential between the connection lines.

The blocking mechanism therein preferably responds to the reversing ofthe main pump as triggering control signal. A preferred embodiment ofthe invention envisions that on at least two ends of the drive cylindera position indicator responsive to the passage by of the piston isprovided for generating the control signal. Thereafter the blockingmechanism is, time delayed after completion of the reversal process,again deactivated. This means, that the blocking mechanism responds forexample to a time delay element or a control signal produced by areversal process of the main pump with de-activation of blocking.

A preferred embodiment of the invention envisions that the blockingmechanism is provided spring biased, preferably in the open position, inthe flushing line downstream of the flushing oil pressure limitingvalve. Basically it is also possible that the blocking mechanism is aone-way blocking valve provided in the flushing oil line between thereversing valve and the flushing oil pressure limiting valve.

A third possible solution envisions that the blocking mechanism includesa two-way blocking valve provided in the flushing branch upstream of thereversing valve.

It is in principle also possible that the blocking mechanism includesone pressure relief valve respectively in each of the two control linesof the reversing valve in communication with the connection lines. Thepressure relief valves are simultaneously urged into their reliefposition upon activation, so that the control pressure (pre-controlledpressure) at the reversing valve is released towards the tank. In thiscase the same pressure exists on both sides of the reverse valve, sothat the valve pusher or slider is brought via a spring centering into acentral position and thereby the flushing branch is blocked.

BRIEF DESCRIPTION OF THE FIGURES

In the following the invention will be described in greater detail onthe basis of the illustrative examples shown in the figures. There isshown in:

FIG. 1 a circuit diagram of a drive hydraulic for a two-cylinder thickmatter pump with closed hydraulic circuit and integrated flushing branchaccording to the prior state-of-the-art;

FIG. 2 a circuit diagram of a drive hydraulic according to FIG. 1 with afirst blocking variant in the flushing branch;

FIG. 3 a circuit diagram of a drive hydraulic according to FIG. 1 with asecond blocking variant in the flushing branch;

FIG. 4 a circuit diagram of a drive hydraulic according to FIG. 1 with athird blocking variant in the flushing branch;

FIG. 5 a circuit diagram of a drive hydraulic according to FIG. 1 with afourth blocking variant in the flushing branch;

FIG. 6 a measurement diagram showing particular actual condition valuesof a drive hydraulic according to FIG. 1 (state-of-the-art) during thereversing process of the main pump over time;

FIG. 7 a measurement diagram showing particular actual condition valuesof a drive hydraulic according to FIG. 2 (invention) during thereversing process over time.

FIGS. 1 and 5 show the basic circuit diagram for a drive hydraulic for atwo-cylinder thick matter pump with a closed hydraulic circuit and aflushing branch.

The basic circuit includes a hydraulic pump 10 embodied as a reversingor adjustable pump, to the outlets 12, 14 of which piston-cylinder unitsembodied as drive cylinders 20 and 22 are connected via respectively oneconnection pipe 16, 18. In the illustrated embodiment the pumpconnections 24, 26 are respectively located on the rod end of the drivecylinder. On their opposite end the drive cylinders are connected witheach other via an oscillating hydraulic line 28. A pistons 30, 32, withthe associated piston rods 34, 36 of the drive cylinders 20, 22, arealternatingly driven in counterstroke via the main pump 10. For this, apivot disk 38 of the pump is so reversed via a not shownhydro-mechanical or electro-mechanical control mechanism within the mainpump that the high pressure side and the pre-tensioned low pressureside, during each of the reversing processes, change over to the otheroutlet 12,14. This means, that the connection lines 16, 18 arealternatingly acted upon with high pressure (HP) and pre-tensioned lowpressure (LP).

The hydraulic circuit further includes a feed pump 42, motor driven viathe same drive shaft 40 as the main pump 10, of which the suction sideis in communication with the oil tank 44 and the pressure side 46 is incommunication with the connection lines 16, 18 via feed check valve 48.The pressure side 46 of the feed pump 42 is, besides this, limited to alow pressure level (for example 34 bar) via a pressure limiting valve50. The feed pump therewith has the task of maintaining a pre-tension onthe low pressure side of the closed hydraulic circuit, as a result ofwhich the main pump 10 is continuously supplied with hydraulic fluid onthe low pressure side.

The basic circuit according to FIGS. 1 through 5 further includes aflushing branch 52, which on the inlet side is connected via a two-wayvalve or alternating check valve 54 to the connection lines 16, 18, andcommunicates, via a low pressure limiting valve 56 and the heatexchanger 58 of a cooling equipment assembly, with in the oil tank 44.The alternating check valve 54, embodied as a 3/3-way valve, is incommunication with the hydraulic connection lines 16, 18 via itspre-control inlets 60, 62, in such a manner that the slider is pushed tothe respective low pressure side via the currently prevailing pressuredifferential (Δp=HP−LP). In a special case one such slider is redirectedor rerouted with a pressure differential of approximately 4 bar againstthe force of the centering springs 64. The pressure limiting valve 56 inthe flushing branch 52 is likewise at the low pressure level. Thepressure setting there is, for example, 4 bar lower than the LP-limitvalve of the feed pump 42.

In the differential pressure neutral area within the 4 bar range, thereverse valve 54 switches to the central or middle position, which isthe blocking position of the flush branch of the two connection lines16, 18.

The measurement diagram according to FIG. 6 illustrates the timesequence of certain condition values of the drive hydraulic according toFIG. 1 (state-of-the-art) during the reversing process of the main pump10. The individual time-dependent curves illustrated individually in thediagram: pivot angle, pressure at connection pipe A and B, supplypressure, and flushing oil amount, are all respectively provided withtheir own ordinate scale.

In the measurement section shown in FIG. 6 the last pump stroke ends attime point 76.52 sec. Until then, the connection pipe B works under highpressure (HP-B) and the connection pipe A under low pressure (LP-A).Then the pivot angle of the pivot disk is pivoted with high speedthrough the zero position in the time between 76.52 and 76.57 sec and bytime 76.67 to a push-over-point at approximately 12 mm. Almost withoutdelay the low pressure first rapidly increases at connection pipe Awhile the high pressure on the side B, as expected, drops. It is notablethat with the increase in low pressure, from the beginning of thereversing process on, a very high flush oil stream is produced, of whichthe peak greatly exceeds the illustrated scale in the time between 76.52and 76.83 sec. As a result, there is not an availability of sufficientoil in the new low pressure side B for a building up of pressure, inorder to build up sufficient pressure on the high pressure side A viathe high pressure pump. The result is a collapse in low pressure in lineB which, in the time frame 76.77 through 76.81 sec, drops all the way tozero. A consequence thereof is the occurrence cavitations in the area ofthe main pump. Since the supply pressure also drops to values of lessthan 10 bar (instead of 30 bar), pump banging can also occur there. Aconsequence thereof is substantial pressure oscillations also in thearea of the new high pressure side A, which can lead to pressureslamming and friction wear in the pump area.

In order to avoid the disadvantages in the conventional operation of thedrive hydraulic as can be seen from FIG. 6, precautionary measures aremade in the illustrative embodiment shown schematically in FIGS. 2through 5, with which the dangerous collapse in low pressure can beeliminated. In all four embodiment variations the flushing oil stream isinterrupted during the reversing process with the aid of a suitableblocking mechanism.

In the embodiment according to FIG. 2 the blocking mechanism isconstituted by a one-way blocking valve 66 located in the flushingbranch 52 downstream of the low pressure limitation valve 56, of whichthe control input 68 prior to each reversing process is triggered by anend position signal initiated by the arriving piston 30 or as the casemay be 32. The initiation brings about a displacement of the blockingvalve slider against the force of the biasing spring 70 in the blockingposition. A not shown timing element or a further initiating signalreturns the blocking valve 66 again to the flow through position shownin FIG. 2 in which the flushing stream from the instantaneous lowpressure side is brought into operation towards the oil tank 44.

The effect of the blocking valve 66 can be clearly seen from the diagramshown in FIG. 7. In this diagram the two time points 36.54 sec and 36.90sec are recognizable, in which the blocking valve 66 is closed and isagain opened. The closing of the blocking valve starts several tenths ofseconds prior to the beginning of the reversing process at time point36.60 sec. Due to the inertia of the system a small peak results in theflushing stream during the increase in the low pressure in connectionpipe A, which however rapidly drops and can be prevented by the somewhatearlier closing of the blocking valve. It is however clearlyrecognizable, that the oil amount of the feed pump is here sufficient toavoid a pressure collapse on the low pressure side. Also, the supplypressure remains nearly constant. The low pressure side remainssufficiently pre-tensioned over the entire reversing process so that thepressure build up on the high pressure side can occur with a sufficientpush-over effect (peak) in the HP-A curve between 36.9 and 37.0 sec. Dueto the elevated pressure still remaining in line B during opening of theblocking valve 66, there results a short duration surge of the flushingoil amount, which in this position however has no effect on the pressurebuild up in the connecting connection lines. A shift of the opening timeof the blocking valve to a later line point in time could, if desired,further reduce this flush oil amount.

In FIGS. 3, 4 and 5 additional embodiment variations are schematicallyshown for a blocking mechanism in the flushing oil branch, which producesimilar result effects to the illustrative embodiment according to FIG.2 in the time sequence of the condition parameters in the sense of FIG.7.

Accordingly in the illustrative embodiment shown FIG. 3 a one-wayblocking valve 72 of similar construction to FIG. 2 is provided in theflushing oil branch between the reversing valve 54 and the limitationvalve 56. A precondition for this is that the parts 54 and 56 areseparate components, between which space remains for the blocking valve72. The blocking valve 72 is brought into its blocking position via thecontrol input 74 against the force of the return spring 76.

Also in the case of FIG. 4 a two-way blocking valve 78 is in theflushing branch 54, this time upstream of or ahead of the reversingvalve 54, which on the inlet side is connected with the two connectionlines 16 and 18. Similar to the case of FIGS. 2 and 3, this blockingvalve is also displaced via a control input 80 against the force of thespring 82 at the beginning of the reversing process into the blockingposition, and after expiration of a pre-determined time is returned viathe spring 82 into its through-put position.

In the illustrative embodiment shown in FIG. 5 the blocking function inthe center position of the blocking valve 54 is used for blocking theflushing branch. For this purpose, pressure relief valves 84 areprovided in the pre-control lines of the reversing valve 54, which viathe control inputs 86 are controllable, thereby bring the reversingvalve into its differential pressure neutral middle position against theforce of a pressure spring 88 in which the flushing branch 54 isblocked. The deactivation of the pressure relief valve occurs again timedelayed, after the main pump 10 had reversed with its pivot disk 38 intothe opposite conveyance direction.

In summary the following can be concluded: The invention is concernedwith a hydraulic drive, in particular for a two-cylinder thick matterpump. The hydraulic drive includes at least one main pump 10 embodied asa hydraulic reversing pump as well as two hydraulic actuated drivecylinders 20, 22, which, via pump connections provided at their onecylinder end are connected respectively with one connecting pipe 16, 18of the main pump 10, forming a closed hydraulic circuit, and which atthe cylinder ends lying opposite to the pump connections 24, 26communicate with each other via a oscillating hydraulic line 28. Thehydraulic drive further includes a feed pump 42, with which pressure oilis supplied out of the oil tank to the hydraulic circuit on the sidewhich at the time is the low pressure side, as well as a flushing branch52, via which a flushing stream is branched off from the instantaneouslow pressure side with limited pressure into the oil tank 44. In orderto avoid low pressure collapses during the reversing process of the mainpump 10, the flushing stream is blocked and is then opened again after atime delay following the respective reversing process of the main pump10 independent of the pressure differential in the connecting connectionlines.

1. A hydraulic drive for a two-cylinder pump, including two drivecylinders (20, 22) in a closed hydraulic circuit alternatingly actedupon in counter-stroke with hydraulic fluid under high pressure (HP) andlow pressure (LP), an oil tank, a feed line for feeding a fresh streamof oil with limited pressure from the oil tank (44) into the side whichis currently the low pressure side of the hydraulic circuit, a flushingline for a flushing oil stream branched off from the current lowpressure side and discharging into the oil tank, wherein during eachreversal process of the main pump (10) the flushing oil stream isblocked for a period of time independent of the pressure differential inthe connection lines (16, 18) of the main pump (10).
 2. A hydraulicdrive according to claim 1, wherein two hydraulic actuated drivecylinders (20, 22) embodied as piston cylinder units are connected attheir one cylinder end, via pump connections (24, 26), with respectivelyone connection line (16, 18) of the main pump (10), forming a closedhydraulic circuit, and at their other cylinder ends communicate witheach other via an oscillating hydraulic line (28), wherein the main pump(10) is connected with a control mechanism for alternatingly reversingthe conveyance direction of the hydraulic fluid, alternatively buildingup a high pressure (HP) and a low pressure (LP) in the two connectionlines (16, 18) in counter stroke, further including a hydraulic feedpump (42), of which the suction inlet is in communication with an oiltank (44) and of which the pressure outlet (46) is limited to a lowpressure level and communicates via a one-way valve (48) with the twoconnection lines (16, 18) of the main pump (10), and wherein the aflushing branch (52) is integrated in the hydraulic circuit, which onthe outlet side, with interposition of a pressure limitation valve (56),communicates with the oil tank (44), and on the inlet side is controlledby a control valve (54), which is controlled by the pressuredifferential existing between the connection lines (16, 18) such that inthe case of a pressure differential it opens the connection to the lowpressure side to allow discharge a flushing oil stream into the oiltank, and in the case of a pressure differential neutral centralposition is preferably blocked, and wherein a blocking mechanism(66;7;78;84), temporarily blocks the flushing stream during thereversing process of the main pump (10) independent of the pressuredifferential prevailing between the connection lines (16, 18).
 3. Ahydraulic drive according to claim 1, wherein the blocking mechanismresponds to a main pump (10) reversing control signal as trigger foractivation of a blocking process.
 4. A hydraulic drive according toclaim 2, wherein the blocking mechanism is deactivated with time delayafter termination of the reversing process.
 5. A hydraulic driveaccording to 2, wherein the blocking mechanism responds by deactivationof blocking to a time delay element or a control signal produced by thereversal of the main valve (10).
 6. A hydraulic drive according to 1,wherein on at least two ends of the drive cylinders (20, 22) a positionindicator responsive to passage by of a piston is provided for providinga control signal.
 7. A hydraulic drive according to claim 2, wherein theblocking mechanism includes a blocking valve (66) provided in theflushing branch (52) downstream of the pressure limiting valve (56). 8.A hydraulic drive according to claim 2, wherein the blocking mechanismincludes a blocking valve (72) provided in the flushing branch (52)between the reversing valve (54) and the pressure limiting valve (56).9. A hydraulic drive according to claim 2, wherein the blockingmechanism includes a blocking valve (78) provided in the flushing branch(52) upstream of the reversing valve (54).
 10. A hydraulic driveaccording to claim 2, wherein the blocking mechanism includes onepressure relief valve (84) respectively provided in the two pre-controllines of the reversing valve (54) connected with the connectingconnection lines (16, 18).
 11. A hydraulic drive according to claim 6,wherein the blocking and/or pressure relief valve (66, 72, 78, 84) isembodied as signal controlled directional valves spring biased in thethrough-put direction and signal controlled in the blocking or pressurerelief direction.
 12. A process for controlling hydraulic drives fortwo-cylinder pumps, comprising alternatingly acting on two drivecylinders (20, 22) in a closed hydraulic circuit with hydraulic fluidunder pressure via connection lines (16, 18) of at least one main pump(10) embodied as a reversible pump, supplying fresh oil from an oil tankunder limited pressure to the current low pressure side of the hydrauliccircuit and discharging a flushing stream branched off from the currentlow pressure side with limited oil pressure into the oil tank, whereinthe flushing oil stream is temporarily blocked and subsequently againunblocked during each reversing process of the main pump (10)independent of the differential pressure in the connection lines (16,18) of the main pump (10).